In a wild new experiment conducted on monkeys, scientists discovered that a tiny, but powerful area of the brain may enable consciousness: the central lateral thalamus. Activation of the central lateral thalamus and deep layers of the cerebral cortex drives pathways in the brain that carry information between the parietal and frontal lobe in the brain, the study suggests.
This brain circuit works as a sort-of “engine for consciousness,” the researchers say, enabling conscious thought and feeling in primates.
To zero in on this brain circuit, a scientific team put macaque monkeys under anesthesia, then stimulated different parts of their brain with electrodes at a frequency of 50 Hertz. Essentially, they zapped different areas of the brain and observed how the monkeys responded. When the central lateral thalamus was stimulated, the monkeys woke up and their brain function resumed — even though they were still under anesthesia. Seconds after the scientists switched off the stimulation, the monkeys went right back to sleep.
This research was published Wednesday in the journal Neuron.
“Science doesn’t often leave opportunity for exhilaration, but that’s what that moment was like for those of us who were in the room,” co-author Michelle Redinbaugh, a researcher at the University of Wisconsin, Madison, tells Inverse.
While the study wasn't conducted on humans, Redinbaugh and her colleagues do suggest that researching the central lateral thalamus might lead to new therapies for people dealing with brain traumas, injuries, or disorders of consciousness.
An understanding of these mechanisms makes it possible to properly target damaged systems in patients with disorders of consciousness, Redinbaugh explains. That targeting, in turn, can "help them live better lives."
Revving the “engine of consciousness”
The team recorded the neural activity of two macaques, a type of Old World monkey. Monkey brains are considered one of the closest animal models to a human brain. Importantly, the team simultaneously recorded activity across multiple areas of the brain.
"Science doesn't often leave opportunity for exhilaration, but that's what that moment was like."
They also studied the animals when they were awake, sleeping, and anesthetized. They also used a large number of small electrodes tailored to the shape of the targeted brain area and mimicked the desired activity of brain cells in the targeted area, Redinbaugh explains.
“This allowed us to directly manipulate consciousness and record changes in communication and information flow with a very high degree of spatial and temporal specificity,” Redinbaugh says.
By taking this fine-tuned approach, the research team was able to narrow down the regions of the brain involved in consciousness more specifically than other studies have done, the team says.
From the get-go, the team expected they might be able to wake animals up with this technique because past studies show brain stimulation can arouse humans and animals, as well as treat diseases like Parkinson's, dementia and multiple sclerosis. But seeing “just how powerful it was” surprised them, Redinbaugh says.
“The animal went from being deeply anesthetized to opening his eyes, looking around the room, and even reaching out for objects within only a few seconds of the stimulation turning on,” Redinbaugh describes. “Shortly after the stimulation ended, he went back into unconsciousness like nothing happened.”
The team repeated the experiment a few minutes later and saw the same result. These stunning effects suggest that there is a reciprocal relationship between two areas of the brain — the central lateral thalamus and deep cortical layers — and this relationship operates like an "engine of consciousness," Redinbaugh says.
The question of consciousness
Diving into the monkey brain brings us one step closer to figuring out how human consciousness — the capacity to experience one’s environment and internal states — operates in the brain.
“Macaque monkeys are the best animal model available to compare to humans, sharing a lot of key similarities in terms of brain structure and function,” Redinbaugh says. “This provides a lot of hope that these results can translate clinically.”
How humans and other sentient beings experience the world — consciously — has long puzzled scientists. The ability to smell a flower or feel a pinprick relies on interactions between brain cells in the cerebral cortex at the brain’s surface and the thalamus, the brain’s core, Redinbaugh explains. But up to this point, scientists haven’t been able to narrow exactly which pathways or structures are involved. Now, this study points other researchers to the central lateral thalamus circuit as a pivotal mechanism.
“There are many exciting implications for this work," Redinbaugh says. "It's possible we may be able to use these kinds of deep-brain stimulating electrodes to bring people out of comas.”
The study could also ensure that patients stay knocked out in surgery or help people with various types of consciousness disorders. But these future applications are way down the line. More studies — in humans, not primates — are needed to explore how the central lateral thalamus operates in the brain. We're still far off from answering the consciousness question.
"Our results point to the importance of specific neural pathways for consciousness, but the research doesn’t end there," Redinbaugh says.
Abstract: Functional MRI and electrophysiology studies suggest that consciousness depends on large-scale thalamocortical and corticocortical interactions. However, it is unclear how neurons in different cortical layers and circuits contribute. We simultaneously recorded from central lateral thalamus (CL) and across layers of the frontoparietal cortex in awake, sleeping, and anesthetized macaques. We found that neurons in thalamus and deep cortical layers are most sensitive to changes in consciousness level, consistent across different anesthetic agents and sleep. Deep-layer activity is sustained by interactions with CL. Consciousness also depends on deep-layer neurons providing feedback to superficial layers (not to deep layers), suggesting that long-range feedback and intracolumnar signaling are important. To show causality, we stimulated CL in anesthetized macaques and effectively restored arousal and wake-like neural processing. This effect was location and frequency specific. Our findings suggest layer-specific thalamocortical correlates of consciousness and inform how targeted deep brain stimulation can alleviate disorders of consciousness.